1. Field of the Invention
The present invention relates to an improvement in an arch type strapping machine. In particular, it relates to a roller unit for feeding and pulling back a band, whereby the band is fed from a chamber through the roller unit to an arch type band guide, and is wound around an article to be packed and pulled back after its top end portion has been secured between an anvil and a right gripper. It also relates to an improvement in a transmission mechanism of driving force in the arch type strapping machine.
2. Discussion of Background
Roller units for controlling band-feeding in conventional arch type strapping machines are disclosed, for example, in Japanese Examined Patent Publication No.97115/1993. A band was fed to and pulled back from a band guide by a driving roller 37 and a counter roller 38 as in FIGS. 9 and 10. Since operations to the band were carried out with a pair of the rollers 37, 38 as shown in FIG. 9, the band had to be driven by rotating the driving roller 37 in a forward or a rearward direction. Accordingly, a speed control for the roller at the time of pulling back and tightening in succession was complicated, whereby there were various problems such that high speed feeding for efficient operations was difficult and a torque at the time of tightening was not sufficient.
Also, in order to solve such disadvantages, it can be considered to use a driving roller having a larger diameter, by which the speed of the band at the time of feeding and pulling back can be increased. However, since a tightening force applicable to the band is substantially changed depending on a contact position of the counter roller to the large driving roller, the counter roller being in forcibly contact with the driving roller to produce a returning force to the band, an adjustment of tension force was difficult. Further, since it took a certain amount of time to decelerate the large driving roller having a large inertia at the transient time from the first tightening operation to the secondary tightening operation which is much stronger, the band suffered from an impact force. Accordingly, an equipment for controlling the roller unit was complicated.